The present invention pertains to load-responsive hydraulic systems in which the effective output of the system is controlled by a difference between the load-actuating pressure and the pressure of the pump. This pressure differential is sensed across the directional control valve as fluid is directed from an inlet port to a work port. Thus the system pressure is maintained at a predetermined value above the load-actuating pressure of the fluid motor; and fluid flow to the motor is proportional to the flow opening through the throttling orifice of the control valve.
In load-responsive systems, it has been common to take pressurized fluid from either the control valve or the fluid motor, at the load-actuating pressure of the fluid motor, and to use this fluid to control the effective output of the pump. If the pump is of the fixed displacement type, the control of the effective output has traditionally been accomplished by the use of a bypass valve controlling the flow of excess fluid from the pump output to a sump; and, if the pump has been of the variable displacement or variable discharge type, a displacement or discharge control has been used to control the output of the pump.
There has been one particular problem with load-responsive systems when used with a fixed displacement pump. It is desirable to maintain a low pump pressure at standby to minimize horsepower loss and heat rise; and yet the low standby pressure has meant a low differential pressure; and a low differential pressure has severely limited the maximum flow that the control valve can supply to a fluid motor.
McMillen, in U.S. Pat. No. 3,631,890 proposed the use of a spring and piston to increase the differential pressure at which the system operates and thereby to increase the ability of a given size of control valve to deliver flow to a fluid motor. This particular device has the inherent limitation of poor response because of the time required to compress the spring. A similar device has been disclosed by Tolbert in U.S. Pat. No. 3,777,773.
The present invention provides a solution for this problem through the use of a bypass valve having three fluid-responsive areas and a directional control valve having special circuitry.